In the process of image acquisition using an optical camera (from scene to image display), the image is degraded by the atmosphere, object or camera motion, the camera system's lens, internal optics and image sensors, processing, color interpolation and further post-processing. Thus, the displayed image may look distorted, defocused, color mixed, etc. In other words, the acquired image does not seem realistic compared to that of the scene of interest.
Optical aberrations are defined as the departures of the performance of an optical system from the predictions of paraxial optics (i.e. the ideal optical approximation of the image formation). Indeed, the light wave front, assumed ideally spherical before the optical system, is modified by it, yielding an aberrated image.
Optical aberrations may be divided into two main categories: monochromatic aberrations and chromatic aberrations. Monochromatic aberrations, which occur even when the light is quasi-monochromatic, are in turn divided into those that degrade the image making it unclear and those that deform the image. On the other hand, chromatic aberrations arise from the fact that refractive index of the lens is a function of frequency or color and therefore different “colored” rays will traverse a system along different paths. The aberrations may be summarized as follows.
Monochromatic spherical aberration occurs in a spherical lens or mirror because such lenses do not focus parallel rays to a single point, but instead along the line containing the center of the lens and the focal point, decreasing the contrast and degrade the details of an image.
Monochromatic coma occurs when a ray bundle originated at an object point is oblique with regard to the lens plane. Then, different rays will focus at different positions in the object plane. In images, the effect of coma produces the so-called coma flare, i.e. the repetition of the same object at different positions and with different magnifications.
Monochromatic astigmatism is the lens aberration in which tangential and sagittal lines are focused at two different points along the optical axis. The image is clearest somewhere between these two points, though edges have a certain amount of inevitable blur.
Monochromatic field curvature corresponds to the inability to bring the centre and the border of the image into focus at the same time, with the border out of focus when the centre is sharply focused and vice-versa. This aberration is closely related to astigmatism.
Monochromatic distortion appears because the transverse magnification is a function of the distance between the image center and the pixel under consideration. In the absence of any other aberrations, images as a whole appear as misshaped, even though each point is sharply focused.
Longitudinal chromatic aberration is such that the redder (i.e. longer wavelengths) components of the white light will be focused further away from the lens than the bluer (i.e. shorter wavelengths) components. This phenomenon produces a color halo around the spot in the generated images.
Lateral chromatic aberration appears in scene points which are not in the center of the image. In this case, the magnification and focus are wavelength dependent. In an image, this will result in a different magnification of every “colored” ray and different sharpening. Succinctly, the image will consist of a continuum of more or less overlapping images, varying in size, color and focus.
In images, there may be a predominant aberration with regard to others. However, the resulting image is typically a compound of all the aberrations. It is to be noted that the aberrations described here correspond to the primary aberrations and that higher-order aberrations may also be present in an image. Nevertheless, primary aberrations are the most representative.
In the prior art, the major amount of work on aberration correction involves modification of the optical system, by adding or deleting optical devices. The principle is fairly simple. For instance, if a positive lens introduces a specific aberration (e.g. certain amount of chromatic aberration), then a negative lens introduces the inverse aberration. Therefore, if these two reciprocal lenses are combined, the aberration should be eliminated. There are, however, many other problems, since the introduction of a new lens modifies other properties of the optical system. Thus, a compromise is needed between the amount of aberration that is eliminated and the properties that are desirable in the optical system.
Correction of optical aberrations has also been done by using software-based techniques. Such techniques are in principal similar to optical corrections, i.e. recovering the ideal light wave front by modifying the wave front generated after passing through an optical system. This can be performed, for example, by using the modulation transfer function (MTF), which characterizes uniquely the optical system. This implies a-priori knowledge on the system, either by means of manufacturer data or by experimental calibration.
An example of a software-based correction can be found in U.S. Pat. No. 7,221,793 to Stavely et al. where spatially-varied demosaicing is used to correct lateral chromatic aberration. A drawback of the methods and systems described in U.S. Pat. No. 7,221,793 is that it requires some a-priori knowledge regarding the optical system.